1. Field of the Invention
The present invention relates to a process for producing optoelectric articles.
2. Related Art Statement
The lithium niobate (LiNbO.sub.3) single crystals and the lithium tantalate (LiTaO.sub.3) single crystals have been expected as materials for optoelectronics. It is known that a thin film of lithium niobate is formed on a substrate made of a single crystal of lithium niobate or the like by the liquid phase epitaxial process.
For example, according to the description in "Appl. Phys. Letters", vol. 26 No. 1 (1975), pp 8-10, a thin film of a single crystal of lithium niobate is formed on a substrate of a single crystal of lithium tantalate by the liquid phase epitaxial process. According to "Mat. Res. Bull", vol. 10 (1975), pp 1373-1377, a thin film of a single crystal of lithium niobate is formed on a substrate of a single crystal of lithium niobate by the liquid phase epitaxial process. According to "J. Appl. Phys.", Vol 70 No. 5 (1991), pp 2536-2541, a thin film of a single crystal of lithium niobate is formed on a substrate of lithium niobate doped with magnesium oxide by the liquid phase epitaxial process.
The film forming method in the liquid phase epitaxial process will be explained. FIG. 5 is a graph schematically illustrating a temperature schedule for a melt in the case of the liquid phase epitaxial process. FIG. 6 is a graph showing a solubility curve of a pseudo-binary system such as LiNbO.sub.3 -LiVO.sub.3. First, lithium niobate (solute) and LiVO.sub.3 (solvent) are charged and mixed together. A saturation temperature corresponding to a melt having a charged composition is taken as "T.sub.0 ". While the temperature of the melt is held at T.sub.1 higher than the saturation temperature T.sub.0, lithium niobate and LiVO.sub.3 are uniformly melted. In FIG. 5, "A" corresponds to this molten state. Then, the melt is led to a supercooled state by lowering the temperature of the melt to a temperature T.sub.4 lower than the saturation temperature T.sub.0. In FIG. 5, "C" corresponds to this supercooled state. The substrate is contacted with the supercooled melt.
Moreover, a crystallinity of the single crystal mentioned above can be estimated by a half width of the X-ray rocking curve According to "J. Cryst. Growth", Vol. 132 (1993), pp 48-60, use is made of a substrate of lithium niobate doped with magnesium oxide, and a thin film of a single crystal of lithium niobate having a substantially same small half width as that of the substrate is formed on the substrate.
Usually, in the process of producing a thin film by the liquid phase epitaxial process, it is important to control a film forming temperature, a film forming time period, a degree of supercooling, and so on so as to control film qualities such as film thickness, crystallinity, surface state, optical properties and so on. For example, as for a film thickness "d" of the single crystal film, the following equation (I) is satisfied. EQU d=(1/Cs.multidot.m)(D/.pi.)[2.DELTA.T.multidot.t.sup.0.5 +(4/3)Rt.sup.1.5 ]tm (I)
where "d" is a film thickness, "Cs" is a density of film, "m" is an inclination of a liquid phase line, "D" is a diffusion constant, ".DELTA.T" is a degree of supercooling, "R" is a cooling rate and "t" is a time. The degree of supercooling (the degree of supersaturation) .DELTA.T is expressed by .DELTA.T=(saturation temperature--film forming temperature). The saturation temperature is determined by a composition of the melt. Therefore, if a composition of the melt is constant, the saturation temperature is also constant. In this case, .DELTA.T is directly determined corresponding to the film forming temperature, and thus the film thickness must be constant from the equation (I).
However, in actual cases, if a film is formed at the same film forming temperature, a film thickness is varied. This is recently understood. Moreover, in this case, the other film qualities such as crystallinity, surface state and optical properties are also varied. Therefore, in the case of forming a single crystal film by the liquid phase epitaxial process, it is difficult to form a film having an excellent crystallinity.
The present inventors have further investigated this point. In the liquid phase epitaxial process, a film forming is performed by maintaining the melt at a temperature higher than the saturation temperature, and then cooling the melt to the film forming temperature lower than the saturation temperature so as to subject it to a super cooling skate. A crystallinity of the film is determined by this supercooling state. In the conventional understandings, since the supercooling state is determined by the saturation temperature and the film forming temperature, it is possible to form stably a film having an excellent crystallinity if controlling them.
However, in actual cases, the supercooling state is affected by various factors, and thus a quality of the single crystal film is not constant, even if the saturation temperature and the film forming temperature are maintained constant. For example, if a concentration of solute of the melt is slightly varied, or if a cooling rate from a high temperature to the film forming temperature is varied, the supercooling state is varied drastically, and thus it is not possible to form a film having an excellent crystallinity. Particularly, if a film is repeatedly formed on the substrate during the actual film forming process, the composition of the melt changes with high response. Consequently, it is difficult to keep the concentration of the solute constant. Therefore, it is difficult to form films with high reproducibility.
In particular, the reproducibility becomes poor, with deteriorated crystallinity contrary to the expectation, in a film forming temperature of more than 1000.degree. C. in which the single crystal film having an excellent crystallinity must be inherently formed.
Moreover, single crystal substrates made of lithium niobate are now produced by the Czochralski process. However, in this process, on a single crystal substrate, it is difficult to form a single crystal film having a more excellent crystallinity than that of the single crystal substrate. Consequently, if an optical waveguide substrate, SHG device, and so on are formed on the single crystal film, an optical damage resistivity of the optical waveguide becomes poor, and an energy threshold level of light which can be transmitted through the optical waveguide is low. Therefore, the substrate produced by the Czochralski process is worked to obtain the optical waveguide substrate, and thus the single crystal film produced by the liquid phase epitaxial process is not used effectively for the articles mentioned above. This problem is expected to be solved.